Facsimile synchronizing system



March 11, 1941. H. c. RESSLER 2,234,919

FACSIMILE SYNCHRONIZING SYSTEM Filed Aug. 15, 1958 3 Sheetsheet l March11, 1941. H,

C. RESSLER FACSIHILE SYNCHRONIZING SYSTEM Filed Aug. l5, 1938 5Sheets-Sheet 2 Kessler March 1l, 1941. H. c. Rr-:ssLER FACSIMILESYNCHRONIZIHG SYSTEM Filed Aug. 15, 1938 -3 Sheets-Sheet 3 |...dlllfl1INVENTOR vzfzy: Passief Patented Mar. 11,4'1941 PATENT OFFICE FACSIMILE.SYNCHRONIZIN G SYSTEM Hugh C. Ressler, Richmond, Staten Island, N. Y.,assignor to Radio Inventions, Inc., New York, N. vY., a corporation ofNew York Application August 15,

11 Claims.

My present invention relates to methods of and means for synchronizingfacsimile recorders and the like with a desired transmitter.

One object oi my present invention is to pro- 5 vide means forgenerating an alternating current synchronized by impulses transmittedto a facsimile receiver and suitable for operating a synchronous motorto drive a facsimile recorder.

A particular object is to provide apparatus in the strength of thereceived synchronizing impulses increases.

Another particular object is to provide a synchronizin'g system whichchanges from a passive driven system in the presence of strongsynchronizing impulses to an active self-sustaining system in theabsence of synchronizing impulses.

Still another particular object is to provide means for convertingrectangular impulses into damped oscillations, of applying these dampedoscillations to a tuning fork for synchronization purposes and ofutilizing the output of the tuning fork for driving a synchronous'directcurrent' to alternating current converter the output of which isutilized to drive a facsimile recorder synchronous motor.

These and other objects will be apparent from the detailed descriptionwhich is given in describing the various figures of the drawings.

In the past, facsimile systems have been synchronized either bytransmitting a synchronizing signal along with the picture signal or byemploying very stable frequency control means at both transmitter andreceiver. In the former type of system the synchronizing signals wereused to control the receiving apparatus with a fixed and usuallycritical degree of control. Too strong a degree of control, in general,causes nonlinear distortion of the scanning lines and too i weak controlpermits the system to lose synchronism. Accordingly changes in signallevel, especially those caused by fading in a long radio circuit havesuffered from alternate distortion and loss of synchronism. In thelatter type of syste-m the control apparatus is expensive and even themost elaborate systems drift over long periods of time and hence requireconsiderable attention. The present invention overcomes thesediiiiculties by employing a relatively stable system with automaticcontrol of both stability of the local system and automatic degree oicontrol by received synchronizing signals.

Briefly, my present system operates in the manner described below. Thetransmission of faci simile signals is accomplished by cyclic scanningwhich the stiffness of synchronization increases as' 193s. serial No.224,363 (ol. 17a-69.5)

of an object. A single scanning cycle for instance may last for sixtenths of a second and comprise one elemental strip of a subject to betransmitted. The actual picture signal may occupy four tenthsof a secondor sixty-seven percent of a. cycle. 'I'he remaining two tenths of asecond or thirtythree percent of a cycle may be utilized for picturemargin, synchronizing signal and the like. I prefer to use impulses forsynchronizing signals oi' either rectangular shape or of a shape to con-10 tain a maximum component of the desired synchronous frequency. iThese synchronizing impulses are generated in fixed relation to portionsof the transmitterA scanning cycle.

In the receiver the synchronizing impulses are impressed upon a systemresonant to the desired synchronous frequency. The desired synchronousfrequency may be 60 cycles suitable for driving a synchronous motorwhich in turn drives the receiver recording mechanism. The resonant sys-2o tem may be a tuned electrical circuit or it may have a mechanicalresonance coupled thru an electrical system as for instance in the caseof a. tuning fork.

Received synchronizing'impulses are impressed 25 upon the electrical orelectro-mechanical resonant system and by shock excitation produceddamped trains of synchronous frequency voltage. The clamping of thedamped oscillations may be decreased by using a resonant system with ahigher Q, i. e. ratio of Lw to R or the eiective resistance of thesystem may be reduced by positive feed back or a negative resistancedevice in shunt.

The synchronous voltage ,produced in this way is used to operate a reedor interrupter in a direct current to alternating current converter thusproducing a source of synchronous alternating current power suitable fordriving a synchronous motor at the receiver recorder.

In a preferred form of my present system a o tuning fork is coupled to athermionic vacuum tube amplifier in such a way that energy is fed backfrom the output of the amplifier to the input thru the mechanicalimpedance o! the tuning fork. The feed-back circuit is proportioned and5 coupled in such a way that it is regenerative and may be made toincrease the Q oi.' the tuning fork or generate sustained oscillationsby varying the amplifier gain as by varying the amplier tube bias. Whensynchronizing impulses are fed chronizing impulses serve to synchronizethe frequency of oscillation. The strongest synchronization takes placejust below the point of self-oscillation. By rectifying either theamplifier output or the received impulses and' using 'the filteredrectier output to control the ampliiier gain, the system may be made tochange from a passive system generating damped oscillations or tightlycontrolled synchronous oscillations to an active system generatingsteady unsynchronized oscillations depending on the amplitude ofsynchronizing impulses present in the input. In this way the systemoperates asa tightly synchronized driven system in the presence ofstrong synchronizing impulses, as a weakly synchronized self-oscillatingsystem in the presence of weak synchronizing impulses and as a stablenon-synchronized selfoscillator in the absence of synchronizingimpulses. This mode of operation is particularly valuable in connectionwith long radio circuits where fading may destroy the synchronizingimpulses for short periods of time, but since the system then becomesself-sustaining only small drifts will take placebei'ore theY signalreturns. This system thus has the advantage that the degree of controlof synchronizing signals depends on their amplitude whereby the systemis not critical in adjustment and does not lose lsynchronism onmomentary loss of signal.

The synchronizing signal sent out from the transmitter may be a group ofwaves of the desired frequency which, except for a transient term, areessentially sine waves. In the receiver this synchronizing signal may befed to the regenerated tuning fork. The tuning fork when made lesssensitive to external signals is, of course, more stable.Thesynchronizing signal, consisting of several cycles of the desiredfrequency, acts to correct the tuning fork frequency a slight amountduring each cycle of synchronizing signal. Since these are severalcycles of correcting signal, the fork frequency may be made very stable,minimizing disturbing effects of stray signals and yet maintaining goodaverage synchronism.

lAnother mode of operation is to transmit on a radio carrier or over aline substantially rectanguiar impulses which may be modulations ofsubcai'rier signal for synchronization purposes. In

the receiver these impulses are rectified producing unidirectionalrectangular impulses or corresponding groups of sub-carrier signalswhich are impressed on an. electrical circuit tuned to the desiredsynchronous frequency. The im'- pulses by shock excitation generatedamped wave trains of synchronous frequency voltage which may be used tosynchronize the tuning fork as described above for groups of sine wavesynchronizing.

In the drawings: v

Fig. 1 shows a circuit diagram of the present synchronizing system.

Fig. 2 shows a modification of the present synchronizing system. l

Fig. 3 shows various signal waves useful in explaining the operation ofthel synchronlm'ng system.

Fig, 4 shows `further explanatory waves.

Fig. 1 shows a circuit of one form of the presentA synchronizing system.A cascade two stage amplier employing thermionic vacuum tubes I and 2 isconnected with its input across a tuned electro-magnet I0 and its outputacross a second tuned electro-magnet I2. Electro-magnets I0 and I2 arecoupled by coupling each magnetically to a tine of the tuning fork 9.The amplitude of oscillation or the amount of regeneration thus producedis'controlled by rectier 3. The generated signal controls a directcurrent to alternating current converter by means of amplier tube 4. Thealternating current output of the converter is used to operate thesynchronous motor 66 which turns the mechanism of the facsimilereceiving scanner 68. The facsimile receiver 5 supplies picture signalsto the scanner 68 and synchronizing signals to the tuning fork throughthe use of electromagnet 8.

The operation of the system of Fig. l will now be traced in detail.While 'the system may be operated from a wire line signal, a radiosystem will be described. Radio signals modulated with facsimile picturesignals and synchronizing signais are picked up on antenna 'I andamplied land detected by the facsimile receiver 5. Picture signals fromthe receiver are applied to receiving scanner 68 over leads 86 and 8'Iand synchronizing signals are applied to tuning fork 9 by means ofelectro-magnet 8.

The cascade amplifier, including thermionic vacuum tubes I and 2, isused to regenerate tuning fork 9 and may be operated to causeselfsustaining oscillations. Tube I includes heater 22, cathode I'I,controlgrid I8, screen grid I9, suppressor grid 20 and plate 2I. A platebattery 43 is shown energizing electrodes of tubes I and 2- althoughother well known means may be used. Heater voltage supply for heaters22, 36, 52, 89 and is not shown, but will be understood to be any wellknown means. Plate 2| is energized from battery 43 through thedecoupling resistor 88, interstage coupling transformer 30 and platecurrent meter 33. Screen I9 is energized from the voltage supply 43through the decoupling resistor 88 and the screen dropping anddecoupling |resistor 28. Cathode I'I receives a bias from the voltagedrop across resistor 25 due to the cathode current plus the currentthrough resistors 23 and 24. The plate voltage is by-passed by condenser2l and the screen voltage is Icy-passed by condenser 26. The voltageacross electro-magnet I0 tuned by condenser II is applied to grid I8.Coupling transformer 29 is connected with its primary connected to plate2| and its secondary connected to grid 38 of tube 2. The secondary 3| istuned to the synchronizing frequency by condenser 32. Heater 36 of tube2 energizes cathode 3l. Cathode 3l receives a bias due .to cathodecurrent flowing thru resistor 35 connected to common ground 6 andby-passed by condenser 34. Plate 39 of tube 2 is energized from voltagesource A33 Ithrough primaryv 4I) of output transformer 4I. Outputtransformer 4I receives signal voltage from plate 39 and feeds rectier 3from its secondary 42. Secondary 42 is shunted by resistor 44. Rectifier3 comprises cathodes 4l and 48 energized by heaters 89 and 98 and anodes46 and 45. Lead 9| applies a bias to cathodes 41 and 48 from" the dropacross resistors 24 and 25. Signal voltage across secondary 42 isapplied between anodes 45 and 46 and cathodes 41 and 48. Rectifledvoltage is developed by the charges produced in condenser 50 leakingoft' through resistor 49.'

The anode end of resistor 49 which is connected between anodes 45 and 46and ground 6 will become negative with respect fto ground due to rectiedcur-rents owing in it and this negative voltage is applied to gri-d IBthrough filter resistor I0 and the winding of electro-magnet I0.Condenser 'II cooperates with resistor 10 to smooth the rectied voltageapplied to grid I8. The sigthe gain of the amplifier.

nal voltage across secondary I2 is also applied .to electromguet I2tuned by condenser I3 through control resistors I5 and I5. Inaddltion,the signal across secondary 42 is applied to grid 53 of amplifier l.Amplifier 4 `includes heater 52, energizing cathode 5I, control grid 53and plate 54. Cathode 5I is connected to ground'9 through bias resistor-55 which in -.turn is by-pas'sed by condenser 59. Plate 54 is energizedfrom a plate voltage source 51 which is shown as a battery, but notlimited to such. The plate current of tube l is caused lto flow throughthe interruptor electro- 'magnet 58 and hence causes reed 59 to vibratein .response-to signals applied .to grid 53. The mid-point of winding 55of the auto-,transformer 64 ls connected to one side of a source ofdirect current power by means of lead 93. 'Ihe other side of rthe sourceof direct current power is connected alternately to opposite sides `ofwinding 95 .through reed 59 and Ithe alternating contacts SII and i IThus an alternating current is caused to ilow in winding 65 in step withthe signal voltage applied to .grid 53 since electro-magnet 59 operatesthe reed 59 at this frequency. if no direct current were permitted Itoiiow through electromagnet 5l the alternating frequen-cy would bedoubled. A The synchronous motor 66 is supplied with alternating currentvoltage from transformer` 64 :through taps 92 and 93. "Ihesynchronousmotor -95 is mechanically coupled to drive receiving scanner 69.

In operation, tuning fork 9 is set in vibration by pressing startingbutton I4, causing a surge of current to flow through the winding ofelectromagnet I2. This surge of current magnetizes magnet I2 attractingone tine of fork 9. The motion of this tine induces a motion in theother tine and hence a voltage in the winding of electro-magnet Ill.'I'his voltage is amplified -by tubes Il and 2 and reapplied to fork 9through electro-magnet I2 Fork 9 acts as a mechanical filter and withsufficient gain in the amplifier and proper polarity of electro-magnetsI0 and I2 self-sustaining oscillations will be set up at a frequencyvery close to the natural frequency of the fork-9. Tuningelectro-magnets I0 and I2 by means oi' condensers II and I3 to thefrequency of the fork assists the production of oscillations. After thesystem has started t0 oscillate, button Il is vreleased and a partialcontrol of oscillation amplitude may be affected by adjustment ofcontrol resistance IE in series with fixed control resistance I5. wellknown methods may be employedl for initially setting the amplifier gainas for instance by varying the turns ratio of transformer 29-30--3L Withthe system oscillating an alternating current, voltage will be fed torectifier 3. The rectified direct current produced by rectifier 3 isapplied to grid I8 and here reduces This reduction in amplifier gaintends to reduce the amplitude of oscillation and a stable automaticallycontrolled oscillation amplitude results.

Now, if synchronizing signal voltage of the same frequency a's lthe forkfrequency, or of nearly the same frequency is-applied to fork 9 by meansof electro-magnet 8, the fork frequency may be locked in step with thesynchronizing signal frequency. With large synchronizing signal voltageapplied to electromagnet 9. the system may be adjusted so that it willnot oscillate in a self-sustaining manner at the same amplifier gain ornet bias on grid I3. In this condition synchronization is very tightOther and exact. If now the synchronizing signal is removed due tofading of the radio signal, the amplitude of oscillation of the forkwill start to die out. As it dies out the rectifier 3 receives a smallerand smaller signal voltage andA produces a decreasing bias on grid I8. Apoint is finally reached at which the gain of the amplifier is increaseduntil self-sustaining oscillations are again produced andthe systemcontinues at the natural frequency of fork 9. If fading periods areshort, the system will drift but little until synchronizing signals areagain produced and the system returns to its initial tightlysynchronized condition.

'I'he operation is much the same if synchronizing signals are sent forshort intervals interspaced with the facsimile signal, as for instance,at the end of each line. When the synchronizing signal is received, thefrequency is corrected and, during the picture signal interval, the forkcarries the system along.

The synchronous voltage producedin the fork amplifier controls thedirect current to alternating current converter as described above, and

-runs the synchronous motor 66, and hence scanner 68, in step with thereceived synchroniz# ing signal.

The level of received synchronizing signalat which the self-sustainingcondition of the fork oscillation system takes place may bepredetermined by adjustment of control resistor I6 or by adjustment ofthe amplier gain.

One form of composite picture signal and synchronizing signal wavesuitable for operating the system as described is shown in Fig. 4. Acomplete scanning cycle is represented by synchronizing signal K pluspicture signal I where I represents one line of the picture beingtransmitted. The number of cycles in the synchronizing signal K may bedetermined by practical considerations. 'Ihe larger the number of cyclesthe more stable may be the fork and still by a small correction on eachcycle reach exact synchronism at each synchronizing interval. If thesynchronous frequency is 60 cycles and the synchronizing interval is twotenths of a second, twelve cycles of synchronizing frequency may betransmitted during each interval. The effective-- receives, amplies anddemodulates the facsimile modulated carrier signals and if necessaryseparates synchronizing and picture signals. It will be understood thatas in the case of Fig. l

receiver 5 may be replaced by suitable apparatus for operation from awire line. In Fig. 2 the synchronous motor 66 and receiving scanner 68of Fig. 1 are represented by synchronous motor and receiving scannerblock 85. Similarly the various components of the converter system arerepresented by block 84 and the tuningfork amplifier and rectifiercontrol system by block 33. The tuning fork 9 and its associatedelectromagnets 8, IU and I3 are repeated for purposes of explanation inFig. 2. The circuit of Fig. 2 is especially adapted to operate from arectangular or other impulse type of synchronizing signal as shown at ain Fig. 3, These synchronizing Plate voltage for amplier 12 may besupplied l5 by battery 82 or its equivalent. The damped oscillationsapplied to grid 14 are amplified and applied to fork magnet 8 by placingit in series with plate 13. With a bias, as battery 82, in series withmagnet 8, its operation on tuning fork 9 is at fundamental frequencyWhile without such bias its control will be exerted at twice fundamentalfrequency. Thus, magnet 8 receives damped oscillations of synchronousfrequency voltage instead of the type of signal shown in Fig. 4, but theremainder of the system is op.- erated and synchronizes in a mannersimilar to the system'described in connection with Fig. l.

In both Figs. l and 2 the synchronizing signals are shown. applied tofork 9 by means of a tertiary magnet 8. This method of synchronoussignal application causes the fork to act as a lter selecting thedesired synchronous frequency components and eliminating the picturesignal.

Fig. 3 shows at e a synchronous impulse a space b picture signal c andsecond space d. At f is shown the modulated carrier resulting frommodulation by signals of lin'e e. At g is shown the received andrectified carrier voltages which should be a reproduction of line e. Online h 4.0 is shown the damped oscillations resulting from thesynchronizing impulse a as described in connection with Fig. 2. While nosub-carrier waves have been shown, it will be understood that thepicture signal and synchronizingsignals shown 45 may be impressed on asuitable sub-carrier before transmission and that this sub-carrier,usually a high audio frequency, may be retained to any desired point inthe receiver.

It is not intended to limit the invention to the 50 particularembodiment shown but only to those embodiments falling within the spiritand scope of the invention as -set forth in the appended claims.

I claim:

1. In a facsimile synchronizing system, the combination of,means forreceiving synchronizing signals, a regenerated tuning fork, means forautomatically varying the degree of regeneration as a function of theamplitude of said synchron- 60 izing signals.

2. In a facsimile synchronizing system, the combination of, means forreceiving synchronizing signals, a regenerative tuning fork circuit,means for automatically increasing said regeneration to the point ofself-oscillation in response to synchronizing signals below apredetermined level.

3. In a facsimile synchronizing system, the combination of, means forreceiving synchronizing signals, a regenerative self-oscillatory tuningfork,and means for automatically decreasing the regeneration below thepoint of self-oscillation in the presence of received synchronizingsignals of greater than a predetermined amplitude.

4. In a synchronizing system, the combination Cathode 15 may receive abias thru of, synchronizing signal receiving means, low decrementvibratory means, electricalmeans for automatically varying the decrementof said vibratory means as a function of the amplitude of receivedsynchronizing signals, and means for controlling the frequency ofvibration of said vibratory means in accordance with synchronizingsignals having amplitudes greater than a means coupling said output withsaid fork, elec- I tro-magnetic means coupling said receiver with saidfork, and electro-magnetic means coupling said amplifier output withsaid reed whereby the speed of operation of said receiving scanner isdetermined by signals traversing said receiver.

6. In a facsimile receiving system, means for maintaining synchronismwith a desired transmitter comprising, in combination, a facsimilereceiver, a tuning fork composed of magnetic material, a direct currentto alternating current inverter including a magnetic reed fordetermining the frequency of said alternating current, a receivingscanner, a synchronous motor driven by said alternating current fordriving s'aid receiving scanner, an amplifier including input and outputcircuits, electromagnetic means coupling-'said input and output withsaid fork, electromagnetic meanscoupling saidreceiver with said fork,electromagnetic means coupling said amplifier output with said reedwhereby the speed of operation of said receiving scanner is determinedby signals traversing said receiver, and means for rendering said tuningfork and amplifier a self-sustaining oscillatory system for all receivedsignals belowa predetermined level.

7. In a facsimile receiving system, means for maintaining synchronismwith a desired transmitter comprising, in combination, a. facsimilereceiver, a tuning fork composed of magnetic material, a direct currentto alternating current inverter. including a magnetic'reed fordetermining the frequency of said alternating current, a receivingscanner, a synchronous motor driven by said alternating current fordriving said receiving sca-nner, an amplifier including input and outputcircuits, electromagnetic means coupling said input with said fork,electromagnetic means coupling said output with said fork,electro-magneticmeans coupling said receiver with said fork,electro-magnetic means coupling said amplifier output with said reedwhereby the speed of operation of said receiving scanner is determinedby signals traversing said receiver, and meansV interposed between saidreceiver and said third electromagnetic means for converting at least aportion of said Vreceived signals into damped oscillations.

8. In -a. motor driven facsimile receiving system, a system forsynchronizing said motor with received synchronizing signals includingin combination, an oscillatory device, a controllable regenerativecircuit coupled to said oscillatory device capable of generatingsustained oscillasesamoA vice for operating` said motor.

9. In a motor driven facsimile recording system', a system for operatingsaid motorin synchronism .with a predetermined synchronizing signalincluding in combination, an oscillatory device, a controllableregenerative circuit coupled to said oscillatory device for generatingsustained oscillations at substantially the natural frequency of theoscillatory device at regenerative levels above a predetermined value, a.regeneration control circuit connected to a source of synchronizingsignals for decreasing saldregeneration below said critical =value forampliltudes of synchronizing signal greater than a predetermined value,an alternating current generator foroperating said motor. and cir' cuitsfor controlling the frequency of said yalternating current from'thefrequency of said oscillatory device. f

10. The method lof operating a tuning fork in a regenerative circuit inthe presence or absence of synchronizing signals for controlling thespeed ofa motor which includes synchronizing said fork strongly in thepresence of substantial ainplitudes of synchronizing signal, ofautomatically increasing the regeneration atleast to the point 'ofself-oscillation with said fork in the absence of substantial amplitudesof synchronizing sighal, and of operating 'said motor in synchronisrnwith said tuningtork. t

v1l. I'he method of operating a synchronizingsystem including analternating current synchronous motor.; a direct current to alternatingcurrent converter. and a regenerated tuning fork system which includes.applying synchronizing signals to said tuning fork system, utilizingsaid synchronizing signals to control the average regeneration in saidtuning forl system, utilizing signals derived from said tuning fork tocontrol the frequency of alternating current generated by said directcurrent to alternating current converter. and operating saidsynchronousvmotor from alternating current derived

